A 55-year-old woman with a 7 cm non-invasive thymoma and myasthenia gravis had been treated by extended thymectomy via median sternotomy 29 years ago. A microscopic 0.15 cm thymoma (microthymoma) was incidentally found in the thymus during surgery. Twenty-nine years later, a 5 cm thymoma developed in the anterior mediastinum and was surgically treated. The non-invasive first thymoma, the microthymoma and the non-invasive third thymoma were all classified as type AB thymomas according to the World Health Organization (WHO) classification and showed extremely similar histological findings. We think the mechanism underlying the local recurrence of non-invasive thymomas would be intrathymic metastasis because of their clinical and pathological features.